Hillary Clinton's abrupt departure from a Sept. 11 ceremony in New York after falling ill Sunday and the subsequent disclosure that she is suffering from pneumonia are likely to intensify scrutiny on the Democratic presidential nominee's health and potentially inject a new campaign issue into a race between some of the oldest candidates to seek the White House.

Clinton supporters had long dismissed concerns about her health as baseless, insisting that she only suffered from allergies. But Sunday's incident — along with a video appearing to show Clinton having difficulty standing on her own — will only amplify such questions just as the race enters its final weeks.

The incident also could increase pressure on Clinton, 68, and Republican nominee Donald Trump, 70, to release more information about their health. Clinton has disclosed less than some previous candidates. Trump has released almost nothing.

“This is the kind of thing that voters have a right to understand before they cast a vote,” said Katie Packer, a GOP strategist who says she does not support Trump or Clinton.

“Both Trump and Hillary are elderly. They are obligated to release full medical records and full tax returns to the American people. And the media, party leaders and American people should settle for nothing less.”

The pneumonia diagnosis comes as the campaign enters its most grueling phase, weeks before the first presidential debate and as voters start heading to the polls in states with early voting. Her doctor, who made the diagnosis Friday, has advised Clinton to curtail her schedule. But the Democratic candidate did not appear to skip any planned events over the weekend, including a fundraiser in New York City headlined by Barbra Streisand.

After the incident Sunday, Clinton's campaign said it was reviewing possible revisions to her schedule. She had planned to fly to California on Monday for fundraisers and a taping of “The Ellen DeGeneres Show” the following day.

Clinton's lung infection comes after a tough week for her campaign, with polls showing a tightening of the race against Trump. Then over the weekend, Clinton was forced to partially walk back comments she made referring to half of Trump's supporters as “deplorables.”

The new scrutiny of her health will add to those problems.

“Forty-eight hours ago, this was something for the Flat Earth Society and the birth certificate deniers,” Dan Schnur, director of the University of Southern California's Jesse M. Unruh Institute of Politics, said of the speculation about Clinton's health. “Now, it's a topic of legitimate, mainstream political discussion.”

Although Clinton's opponents will surely make an issue of her having pneumonia, her campaign's relatively quick release of the diagnosis marked a significant step toward transparency for a candidate who has often shied away from disclosing information she considers private.

But some critics complained that Clinton did not reveal the potentially debilitating condition Friday when it was diagnosed.

And after she fell ill Sunday morning, her campaign initially attributed it to “overheating” and waited nearly seven hours — as rumors and speculation about her health swirled — until disclosing the pneumonia.

The video, quickly circulated online and replayed on cable news channels, shows Clinton standing uneasily, her knees appearing to buckle, and needing help to get into her van.

A spokesman said immediately afterward that Clinton left the ceremony at the site of the World Trade Center about 9:30 a.m. local time because she felt unwell. Reporters who were traveling with Clinton were not told about her condition or her whereabouts for 90 minutes after she left the ceremony.

“Secretary Clinton attended the Sept. 11th commemoration ceremony for just an hour and 30 minutes this morning to pay her respects and greet some of the families of the fallen,” spokesman Nick Merrill said in the initial statement released by the campaign. “During the ceremony, she felt overheated so departed to go to her daughter's apartment, and is feeling much better.”

The weather was in the low 80s and humid.

Clinton spent about two hours at Chelsea Clinton's apartment and emerged shortly before noon wearing sunglasses.

“I'm feeling great. It's a beautiful day in New York,” Clinton said before heading to her home in Chappaqua, N.Y.

Her personal physician examined her at her house Sunday afternoon and said Clinton was recovering.

“While at this morning's event, she became overheated and dehydrated. I have just examined her and she is now re-hydrated and recovering nicely,” said Dr. Lisa Bardack in a statement. Bardack said Clinton was put on antibiotics Friday.

Trump was uncharacteristically silent Sunday after news of Clinton's illness emerged.

seema.mehta@latimes.com